Small-scale Atmospheric Turbulence Research Articles

LIDAR for Investigation of the Martian Atmosphere from the Surface

The lidar device as part of the meteorological complex of the ExoMars-2022 landing platform is designed to study Martian aerosol, the planetary boundary layer, and small-scale atmospheric turbulence. A miniature lidar based on a pulsed semiconductor laser and an avalanche photodiode in the photon counting mode will make it possible to obtain aerosol backscattering profiles along a vertical path from 10 to 1500 m during the day and from 15 to 10 000 m at night. In the passive mode, the sky brightness is measured in a narrow spectral range and in a narrow solid angle with a frequency of up to hundreds of hertz. The measured fluctuations can provide information about the turbulence of the daytime atmosphere and its relation to dust activity. In the paper we considered the scientific tasks of the experiment, the program of measurements on the surface of Mars and described in detail the components of the equipment and the features of their work.

LIDAR for Investigation of the Martian Atmosphere from the Surface

The lidar device as part of the meteorological complex of the ExoMars-2022 landing platform is designed to study Martian aerosol, the planetary boundary layer, and small-scale atmospheric turbulence. A miniature lidar based on a pulsed semiconductor laser and an avalanche photodiode in the photon counting mode will make it possible to obtain aerosol backscattering profiles along a vertical path from 10 to 1500 m during the day and from 15 to 10000 m at night. In the passive mode, the sky brightness is measured in a narrow spectral range and in a narrow solid angle with a frequency of up to hundreds of hertz. The measured fluctuations can provide information about the turbulence of the daytime atmosphere and its relation to dust activity. In the paper we considered the scientific tasks of the experiment, the program of measurements on the surface of Mars and described in detail the components of the equipment and the features of their work.

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

Reconstructing East African monsoon variability from grain-size distributions: End-member modeling and source attribution of diatom-rich sediments from Lake Chala

Grain-size analysis and end-member modeling of the clastic fraction of the 25-kyr sediment sequence from Lake Chala, a meromictic crater lake on the lower east slope of Mt. Kilimanjaro, reveal crucial aspects of climate-driven environmental change in equatorial East Africa since the Last Glacial Maximum. The finely laminated sediments of Lake Chala contain only up to 40% of clastic components, the rest are mainly diatom frustules and amorphous organic matter. Measured grain-size distributions were split into six statistically meaningful end members, of which four could be linked to a distinct source and transport mechanism of clastic mineral input: fine aeolian dust from distal sources (EM1), fine catchment runoff (EM2), coarser aeolian dust from proximal sources (EM5) and coarse erosive material from the upper crater slopes (EM6). The two other end members (EM3 and EM4) represented frustules of the two most common diatom taxa in Lake Chala, Afrocymbella barkeri and Nitzschia fabiennejansseniana, which had (partly) survived sample pre-treatment.Temporal variation in normalized abundance of the two dust-derived end members are valuable proxies for past changes in monsoon circulation over equatorial East Africa. During Northern Hemisphere cold periods (e.g., Last Glacial Maximum and Younger Dryas) the Inter-Tropical Convergence Zone shifted southward, enhancing northeasterly monsoon winds in the Lake Chala area and increasing advection of fine dust from the dry Horn of Africa region. Simultaneously, more modest continental heating reduced the prevalence of small-scale atmospheric turbulence, and thus the occurrence of dust devils, resulting in reduced influx of coarse dust from drylands nearby. Conversely, abrupt intensification of the southeasterly monsoon at the onset of the Holocene is recorded by an abrupt increase in the amount of coarse dust delivered to Lake Chala. Temporal variation in the end members representing catchment run-off (EM2) and erosion (EM6) mainly reflect changes in lacustrine sedimentation dynamics associated with major lake-level fluctuation, as evidenced by other paleoenvironmental proxies. Overall this study shows that subdivision of the clastic fraction of lacustrine sediments into statistically robust grain-size end members can provide multiple independent and quantitative proxies which help constrain reconstructions of a region’s multi-faceted climate history.

The Use of Weather Forecasts to Characterise Near-Surface Optical Turbulence

The propagation of optical and electromagnetic waves is affected by small-scale atmospheric turbulence, quantified by the structure parameter of the refractive index. In the atmospheric surface layer, the mean structure parameter \({C_{n}^{2}}\) , as averaged over the large-scale turbulence, relates to meteorological forcings through well-documented relationships. Present-day numerical weather forecast models routinely produce these forcings at the global scale. This study introduces a method where the products of such a model are used to calculate the mean optical turbulence near the surface. The method is evaluated against scintillometry measurements over climatologically distinct sites in Western Europe. The diurnal cycle modulation, and regional and seasonal contrasts, are all reproduced by our predictions. Hence, the present method explains and predicts some essential aspects of the meteorological variability of \({C_{n}^{2}}\) near the surface. The noted discrepancies combine instrumental limitations, site peculiarities, differences related to distinct averaging procedures, and model errors, notably from weather forecasts. The minute-scale fluctuations of the measured scintillation rate are also analysed in the light of the forecast weather conditions. Fair-weather daytime periods consistently show a small short-term variability compared to the nighttime and perturbed weather periods. Thus, this short-term variability appears to have a predictable component.

Radio science investigations by VeRa onboard the Venus Express spacecraft

The Venus Express Radio Science Experiment (VeRa) uses radio signals at wavelengths of 3.6 and 13 cm (“X”- and “S”-band, respectively) to investigate the Venus surface, neutral atmosphere, ionosphere, and gravity field, as well as the interplanetary medium. An ultrastable oscillator (USO) provides a high quality onboard reference frequency source; instrumentation on Earth is used to record amplitude, phase, propagation time, and polarization of the received signals. Simultaneous, coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift. VeRa science objectives include the following: (1) Determination of neutral atmospheric structure from the cloud deck (approximately 40 km altitude) to 100 km altitude from vertical profiles of neutral mass density, temperature, and pressure as a function of local time and season. Within the atmospheric structure, search for, and if detected, study of the vertical structure of localized buoyancy waves, and the presence and properties of planetary waves. (2) Study of the H 2SO 4 vapor absorbing layer in the atmosphere by variations in signal intensity and application of this information to tracing atmospheric motions. Scintillation effects caused by radio wave diffraction within the atmosphere can also provide information on small-scale atmospheric turbulence. (3) Investigation of ionospheric structure from approximately 80 km to the ionopause (<600 km), allowing study of the interaction between solar wind plasma and the Venus atmosphere. (4) Observation of forward-scattered surface echoes obliquely reflected from selected high-elevation targets with anomalous radar properties (such as Maxwell Montes). More generally, such bistatic radar measurements provide information on the roughness and density of the surface material on scales of centimeters to meters. (5) Detection of gravity anomalies, thereby providing insight into the properties of the Venus crust and lithosphere. (6) Measurement of the Doppler shift, propagation time, and frequency fluctuations along the interplanetary ray path, especially during periods of superior conjunction, thus enabling investigation of dynamical processes in the solar corona.

A computational model for the rise and dispersion of wind-blown, buoyancy-driven plumes—I. Neutrally stratified atmosphere

A multi-dimensional computational model for the rise and dispersion of a wind-blown, buoyancy-driven plume in a calm, neutrally stratified atmosphere is presented. Lagrangian numerical techniques, based on the extension of the vortex method to variable density flows, are used to solve the governing equations. The plume rise trajectory and the dispersion of its material in the crosswind plane are predicted. It is found that the computed trajectory agrees well with the two-thirds power law of a buoyancy-dominated plume, modified to include the effect of the initial plume size. The effect of small-scale atmospheric turbulence, modeled in terms of eddy viscosity, on the plume trajectory is found to be negligible. For all values of buoyancy Reynolds number, the plume cross-section exhibits a kidney-shaped pattern, as observed in laboratory and field experiments. This pattern is due to the formation of two counter-rotating vortices which develop as baroclinically generated vorticity rolls up on both sides of the plume cross-section. Results show that the plume rise can be described in terms of three distinct stages: a short acceleration stage, a long double-vortex stage, and a breakup stage. The induced velocity field and engulfment are dominated by the two large vortices. The effect of a flat terrain on the plume trajectory and dispersion is found to be very small. The equivalent radii of plumes with different initial cross-sectional aspect ratios increase at almost the same rate. A large aspect-ratio plume rises slower initially and then catches up with smaller aspect-ratio plumes in the breakup stage. The Boussinesq approximation is found to be valid if the ratio of the density perturation to the reference density is less than 0.1.

Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part II: The O2 filtering technique

The detector of the IMST/SA-CNRS improved Lyman-alpha hygrometer is a solar-blind photomultiplier tube that ensures large signal-to-noise ratio and allows a miniaturization of the sampling volume. This PM tube has a relatively large spectral response and can detect, in addition to the Lyman-alpha line, hydrogen molecular transitions emitted by the discharge source between 120 and 180 nm. In order to avoid a spurious sensitivity to oxygen density fluctuations, an active filtering cell is added to absorb the undesired lines. The instrument sensitivity is calculated with the help of a two-gas, three-spectral-line UV-beam absorption model, fitted to calibration measurements. The technique of the active filtering associated with the three-line absorption model produces the following advantages: (1) reduced sensitivity to oxygen fluctuations, (2) a clear physical meaning to the model terms allowing the sensitivity computation in most conditions, (3) increased signal-to-noise ratio, and (4) in most conditions, improved sensitivity to water vapor compared to the monochromatic Lyman-alpha line.

Lateral dispersion of pollutants in a very stable atmosphere—the effect of meandering

A model based on single particle diffusion is introduced to account for the effect of “meandering” on lateral plume dispersion in a very stable atmosphere. It is assumed that small scale atmospheric turbulence is absent, so that only large horizontal eddies are effective. A formula for the lateral standard deviation σ y as function of observation time, distance from source, mean wind speed, lateral turbulence intensity, and scale of the atmospheric motion is derived. Climatological time series of temperature lapse rates, wind speeds, and wind directions can be used as input to calculate σ y . Meteorological data from Risø and the small island Sprogø have been analysed in order to identify all situations in which the atmosphere is so stable that small scale turbulence cannot exist. The purpose is to see in how many of these situations meandering is also absent. The results show that, as a rule, meandering will be present in a strongly stable atmosphere with low wind speeds. If the dispersion by meandering is not taken into account, estimates of mean concentrations can easily be a factor of 4–6 too high.

Studies of aircraft wake chemistry and dispersion

Use of aerospace technology to study aircraft wakes is reviewed. Although aerospace vehicles are suspected of potentially altering the environment, they have an important role in its study. The discussions focus on methods, because few published results are available. It is shown how aerospace vehicles can be used to provide data for increased understanding of the atmosphere and of aircraft exhaust trails where knowledge is inadequate to evaluate fully the potential impact of the engine emissions. Models of aircraft near field exhaust wakes are characterized by jet, vortex, and dispersion regimes. Wake growth in the jet regime is self-determined and rapid, whereas further spreading is inhibited in the vortex regime because of circulating vortex motion. Wake diffusion in the dispersion regime is initially influenced by aircraft induced turbulence but is dominated later by small scale atmospheric turbulence. Computed fluid mechanical results show the importance of effects such as wake buoyancy, wind shear, turbulence, and traffic corridor exhaust buildup on dispersion of the wake. In the jet regime the exhaust characteristics and thermochemistry serve to illustrate initial chemical changes involving potential pollutant species. From computations including flowfield parameters and more than 100 chemical reactions, histories of representative exhaust species are shown for a jet engine in stratospheric, supersonic flight. The influence of photochemistry on certain products in an undiluted hypothetical wake results in significant chemical changes, thereby illustrating the large local effects which might occur if dispersion were not rapid. Aircraft wake species sampling flights show concentrations of nitrogen oxides and carbon oxide in tens to thousands of parts-per-billion in the near field. These data show that for many species, available instruments are adequate for making near field wake measurements. Problems limiting the applications of aerospace technology to atmospheric studies include chemical reaction rate uncertainties, measurement platform limits and availability, certain gas and aerosol measurement difficulties, and uncertainties in our present understanding of atmospheric processes.

Optimal Choice of Parameters for the Measurement of Small-Scale Atmospheric Turbulence with an Airborne Hot-Wire Anemometer

The paper presents typical spectra of probe motions, turbulence, and electronic noise of an airborne hot-wire anemometer. Their relative amplitudes as a function of aircraft size and velocity, hot-wire length, and turbulence dissipation are discussed. The optimal choice of these parameters in the measurement of small-scale atmospheric turbulence with an airborne hot-wire anemometer is recommended.

Airborne Hot-Wire Measurements of the Small-Scale Structure of Atmospheric Turbulence

An experimental investigation of small-scale atmospheric turbulence is presented. Airborne measurements were made in the vicinity of the meterological tower of Brookhaven National Laboratory. Hot-wire probes mounted on the right wing tip of a Cherokee aircraft were used. The anemometer signals were recorded on an analog tape recorder in the aircraft after differentiation and filtering; they were digitized in the laboratory with a high-speed analog-to-digital converter, and recorded on magnetic tape. All data were processed with an IBM-360. A fast Fourier transform program was used to compute the spectra. The average Kolmogoroff constant computed from the small-scale spectra was α = 2.0. The cospectra and quadrature spectra of the small-scale turbulence were virtually zero. The skewness and kurtosis of ∂ui/∂x (the derivative of a velocity vector with respect to the coordinate in the direction of aircraft heading) ranged from -0.03 to -0.50 and from 4.5 to 14, respectively, at Reynolds number (Rλ) from 2280 to 5330. The spectra of (∂ui/∂x)2 were found to exhibit a -0.3 power law in the inertial subrange. The probability distribution of (∂ui/∂x)2 was nearly a straight line in log-normal coordinates.